It is common to covert AC to DC, which can be achieved by using a rectifier circuit. Generally, industrial and civilian power are supplied through the AC power. Taking civilian power as an example, China uses 220 VAC/50 Hz, the Americas uses 120 VA or 110 VAC, 60 Hz, while the UK uses 240 VAC/50 Hz. Other countries and regions may also vary from one to another. Overall, the frequency is divided into two groups: 50 Hz or 60 Hz, with the operating voltage at about 110V and 220V. AC power is characterized in that the direction and amplitude of the voltage (or current) are periodically changing with time, as shown in FIG. 1.
AC changes sinusoidally with time shown in FIG. 1, it is called an alternating sinusoidal voltage, the time needed for repeating a change is called a cycle of alternating voltage, represented by T. The 220V voltage value commonly known in the art refers to the effective value and the peak voltage is √{square root over (2)} times the effective value, namely:220V×√{square root over (2)}=311.1V
The DC voltage (or current) does not change the amplitude and direction with time. The voltage (or current) that does not change direction but may undergo some degree of changes in voltage or current with time is also referred to as the DC voltage.
For industrial and civilian applications, there are needs for turning AC into DC. The process includes first making the current flow in one direction, called unidirectional conductivity, and then making the amplitude stabilized, called filtering. The process of making the AC power turn into unidirectional power is called rectification.
In the existing art, the rectifier circuit is generally divided into half-wave rectifier, full-wave rectifier, bridge rectifier and voltage doubler rectifier circuit. It may be a single-phase and multi-phase rectifier circuit (such as three-phase). In general, the rectifier circuit means a single-phase rectifier circuit. In fact, the single-phase rectifier circuit can be used as a multi-phase rectifier by using simple techniques known in the art.
FIG. 2-1 shows a half-wave rectifier circuit, if the capacitance CL is not connected, the output waveform is shown in FIG. 2-2, as a pulsating DC. With the capacitance CL connected, the output waveform is shown in solid lines in FIG. 2-3, as a relatively smooth pulsating DC. After the circuit enters a steady state, rectifier diode D1 in FIG. 2-1 conducts only during the time from t1 to t2 in FIG. 2-3, charging capacitor CL. Outside this period, capacitance CL is discharging to load RL. For getting a smoothed DC voltage, a larger capacitance CL is necessary. However, the increased capacitance CL causes a shorter conduction time, i.e., the time from t1 to t2. The charging current is large, and thus the circuit in this short period of time consumes a large amount of the AC input current, causing grid voltage waveform distortions. The reference to this principle can be found in the book “Stable Power Supply”, the 1984 edition, published by People's Post (China), ISBN 15045. Figure 2.4.1 on page 33 of the book fully described this principle.
FIG. 3-1 shows a full-wave rectifier circuit, which is generally not used directly in the rectifier in civilian appliances. It is generally used only after having a transformer to obtain two voltages of the same value but opposite phase (center-tapped). If the capacitance CL is not connected, the output waveform is shown in FIG. 3-2, which is a pulsating DC; when the capacitance CL is connected, the output waveform is shown in solid lines in FIG. 3-3, which is a relatively smooth pulsating DC. After the circuit enters a steady state, rectifier diode D1a in FIG. 3-1 conducts and charges CL capacitor only during the time from t1 to t2 shown in FIG. 3-3, and rectifier diode D1b conducts and charges capacitor CL only during the time from t3 to t4. The diode charges capacitor CL when conducted and in other times, capacitance CL discharges to load RL. To get a smoothed DC voltage, a large capacitance CL is necessary, and the increased capacitance CL causes the conduction time from t1 to t2 and from t3 to t4 becomes very short. The charging current is large and the circuit consumes a large amount of the AC input current in this short period of time, causing grid voltage waveform distortions via the transformer. Figure 24.3 on page 35 of the book “Stable Power Supply” described this principle. The distorted waveform is no longer a sine wave, but higher harmonics, which can turned into base waves via Fourier transform. Higher harmonic wave is a source of interference in the power supply.
FIGS. 4-1, 4-2 and 4-3 show a bridge rectifier circuit. The three representation methods are all commonly used, showing the same the connection relationship. FIG. 4-2 is a simpler representation. If capacitance CL is not connected, the output waveform is the same as shown in FIG. 3-2, which is a pulsating DC; when the capacitance CL is connected, the output waveform is shown in solid lines in FIG. 3-3, which is a relatively smooth pulsating DC. After the circuit enters a steady state, rectifier diode D1a and D1c in FIG. 4-1 and FIG. 4-2 charge CL capacitor only in the time from t1 to t2 (FIG. 3-3) when they are conducting. Rectifier diode D1b and D1d charge CL capacitor only in the time from t3 to t4 when they are conducting. The diodes are charging the capacitor CL only when conducted and, in other times, capacitance CL discharges to load RL. To get smoothed DC voltage, a large capacitance CL is necessary. The large capacitance CL however causes the conduction time from t1 to t2 and from t3 to t4 become very short. Consequently, the charging current is large, and consumes a large amount of the AC input current in the short period of time, causing grid voltage waveform distortions. The third paragraph on page 34 of the book “Stable Power Supply” states: “For the full-wave rectifier capacitor filter case, the reader can analyze it according to Figure 24.3, and this analysis also applies to the bridge rectifier.”
For the above described half-wave rectification, full-wave rectifier, the bridge rectifier, the capacitor's voltage rate must be greater than 1.414 times of the input voltage (i.e., the peak voltage). For the 220V AC input, considering the mains voltage instability, the voltage can often rise to about 264V, and the filter capacitor's voltage rate is required to be greater than the peak voltage 373V. To have a safety margin, the capacitor should be of a grade capable of handling a voltage of 400V or 450V.
In summary, the prior art rectifier circuits described above all require a large filter capacitor in order to obtain a smooth DC voltage. The circuit absorbs electric current from the mains only when close to the peak, a large number of consumer electronics, industrial equipment are all doing so and thus the sine wave voltage grid becomes seriously distorted. FIG. 5-1 shows an example of the voltage waveform recorded in the Guangzhou Huangpu Eastern Zone Industrial Park at 8:17 am, Feb. 24, 2012. FIG. 5-2 shows another example of the voltage waveform recorded in the same place at 8:39, Feb. 24, 2012 when most of the factories are in operating. In order to clearly see the waveform, it was recorded using a rectifier circuit without filter capacitor. As shown in FIG. 5-2, the electricity consumption increased when the factories came into operation in the morning, and the waveform further distorted. In the top of FIG. 5-2, the waveform becomes significantly more flat, which is consistent with the above theoretical analysis.
Currently, the power factor correction circuit has been used to solve this problem (referred to as PFC circuit). In the rectifier circuit, a small “filter capacitor” is used to absorb spikes of interference from the mains, such as 0.1 uF to 0.47 uF. The waveform after being rectified will be consistent with FIG. 3-2. Then, by using BOOST topology switching power supply, the voltage is increased to about 400V DC before supplying the power to other circuitry. This is to achieve a high power factor, and avoid the grid voltage waveform distortion.
FIG. 6 shows a method of using the SCR technology to avoid consuming large current when close to the peak. When SCR is used behind the rectifier circuit, the resulting waveform is shown in FIG. 6-1, shaded area 100 means the output area. The drawback of this method is that it does not work with large capacitive loads, and can only work at the descending part of the half-wave. For the reason why it cannot work with large capacitive loads, please refer to the book “Stable Power Supply” (between “3. Inverted L-shaped filter” on page 38 and “4. π-type filter” on page 40).
When SCR is used between the thyristor rectifier circuit and the AC power, the resulting waveform is shown in FIG. 6-2. The current triggering technology however will cause the trigger point asymmetry between the positive half cycle and a negative half cycle, which is shown in FIG. 6-2, where shaded areas 101 are not equal. The drawback of this method is that it does not work with large capacitive loads, and can only work at the descending part of the half-wave.
In low power applications, the PFC circuit is not commonly used due to its higher cost. Thus, in low power applications, AC to DC rectifier circuits are still commonly used which sink large amounts of current from the AC mains when close to the peak, leading to the grid voltage waveform distortion.